Storm demands the US's biggest effort

AUTHORITIES have carried out more than 13,000 rescues in the Houston area as the flooding crisis in America's fourth largest city worsens.

The storm is the heaviest tropical downpour in US history and is estimated to have impacted seven million people across the country's south.

At least 18 people have been killed but that could rise once floodwaters start to recede.

Six members of a single family are feared dead after their van sank into Greens Bayou in East Houston.

The mayor of Houston announced an indefinite midnight to 5am curfew amid reports of looting, armed robberies and people impersonating police officers.

During a visit to Texas, President Donald Trump signalled he would be asking Congress for a huge amount of money to aid the victims of Hurricane Harvey.

"It's going to be a costly proposition,” Mr Trump said in Austin, suggesting the Harvey effort would surpass anything in history.

"Probably there has never been anything so expensive in our country's history - we've never done anything so historic in terms of damage and in terms of ferocity as what we've witnessed with Harvey.

"The sad thing is that this is long term, nobody has ever seen anything this long and nobody has ever seen this much water.”

As rescue and recovery efforts continue, it is too early to say how much damage has been done, but estimates are in the tens of billions of dollars, with the final tally perhaps as high $125 billion, David Havens, an insurance analyst at Imperial Capital in New York, has told Bloomberg News.

About 20% of US oil refining capacity has been halted by Harvey, raising fears about lack of petrol and sending petrol futures to a two-year high this week.

The hurricane was downgraded to a tropical storm after making landfall.